Valve today officially introduced the Steam for Mac. The popular game content distribution and multiplayer game networking platform will bring a wealth of casual games as well as games developed by Valve to Mac users. To celebrate this launch, Valve is giving away a free copy of Portal to anyone who downloads Steam between today and May 24, 2010. Existing steam users who don't have the game yet can simply click on the install button on the game's Store page.

"The Mac is a great platform for developers," said Gabe Newell, president of Valve. "And it's also great that we now have the ability to connect with Mac gamers via Steam. This is the beginning of a groundswell of games coming out for and fully utilizing the openness and power of the Mac." Each Wednesday a new group of games highlighting an area of Steam functionality will be released. This week's titles, which show off Steam Play, include: